Among the various components of a submarine pipeline, the vertical section known as a riser is critical to managing the pipeline. This section connects the piping that runs along the bottom of the sea with the floating production platform.Hung on the platform in deep waters, the risers are subject to extreme operational conditions such as high loads and subsea currents. Corrosion, fatigue, wear, and third-party dam age have to be taken into account to avoid compromising oil and gas production. The flexible pipeline, a solution used in the majority of riser installations, provides high reliability and low maintenance. However, despite advances in design and installation, riser inspection does not comply with operator requirements.
The basic system concept of an autonomous underwater riser inspector (AURI) uses the riser as a guide. The AURI controls its own speed and is designed to transport various types of screening devices. The first AURI was designed to perform a visual inspection with a codified photographic system covering 100 percent of an external riser’s surface. It was the first robot in the world to independently inspect oil pipeline platforms.
The System Prototype
The AURI was created to completely inspect the riser from the touchdown point surface, which is the end of the catenary curve near the sea bottom. Once the tool starts, it inspects without human assistance, automatically returning to the surface when the mission is complete. The AURI is independent in energy supply and control requirements. As there are no cables, the vehicle operates in deep waters (1,000 m) and ultradeep waters (3,000 m). The AURI uses the riser as a guideline, making the tool immune to streams of water.
The first AURI prototype can reach depths of up to 1,000 m. It uses two electric propellers to move along the riser, and possesses various security mechanisms for tool recovery in case of a fault in the system to minimize the chance of pipeline damage.
The mission is defined as the maximum depth to inspect, and is complete when one of the following conditions is reached: (1) the depth sensor achieves the maximum pressure, (2) the race length is achieved, (3) the system exceeds maximum mission duration, or (4) the inclinometer detects maximum inclination. Thanks to its positive buoyancy, the AURI always returns to the surface, even if the electronic system fails or the batteries are depleted.
The first challenge was designing a mechanism with wheels to guide the vehicle through the riser while adapting to diameter changes, swelling, and obstacles. This mechanism also needed to be easy for a diver to open for installation and recovery. The AURI chassis had to be strong enough to bear the weight of all electromechanical pieces and the water stream (which should be weak to prevent any risk to the riser) during the mission. Breaking points were added to ensure a predictable failure in case the AURI chassis became stuck between the riser and the sea bottom.
To obtain positive buoyancy, special floats were designed using high-density resin with glass beads. The floats were molded on top of a structure made of aluminum and covered with thin plates of glass fiber. The floats were incorporated into the vehicle’s structural elements, maintaining a lower density of less than one. The structural elements are carbon fiber bars that unite the floats and form a structural fuselage.